This is the first application filed for the present invention.
Not Applicable.
The present invention relates to optical communications systems, and in particular to a method and system for dynamic broadband compensation of polarization dependent loss within an optical communications system.
Optical communications systems typically include a pair of network nodes connected by an optical waveguide (i.e., fiber) link. Within each network node, optical signals are converted into electrical signals for signal regeneration and/or routing. Exemplary network nodes of this type include Add-Drop-Multiplexers (ADMs), routers, and cross-connects. The optical link between the network nodes is typically made up of multiple concatenated optical components, including two or more (and possibly 20 or more) optical fiber spans (e.g., of 40-60 km in length) interconnected by optical (e.g., Erbium doped fiber) amplifiers.
The use of concatenated optical components within the link enables improved signal reach (that is, the distance that an optical signal can be conveyed before being reconverted into electrical form for regeneration). Thus, for example, optical signals are progressively attenuated as they propagate through a span, and amplified by an optical amplifier prior to being launched into the next adjoining span. However, each optical component introduces polarization dependent effects, which may be manifested as either polarization dependent loss (in the case of filters, isolators, and fiber), or polarization dependent gain (in the case of optical amplifiers). Furthermore, within discrete optical components such as filters, isolators and amplifiers, the polarization dependent effects are typically a function of wavelength. Within optical fiber, polarization dependent losses are a function of wavelength, but may also vary with stress, bending radius, and vibration of the fiber.
When considering the effects of polarization dependent loss/gain on an optical signal, it is convenient to consider the polarization dependent effect (PDE) as a vector quantity, and this terminology is used herein. A more rigorous treatment of PDE is provided in xe2x80x9cPolarized Lightxe2x80x9d (Edward Collett, ISDN 0-847-8729-3). Therefore, when multiple optical components are concatenated to form an optical link, the PDE exhibited by the resulting system is the vector sum of the polarization dependent effects introduced by each of the various components, transformed by the polarization coupling between successive elements along an optical fiber route. Because the PDE of fiber is affected by environmental conditions (e.g. temperature and mechanical stress), and optical switches within the network can dynamically change the topology of a connection, the vector sum of PDE will tend to be a bounded statistical entity changing at a rate of up to tens of Hz.
Optical communications systems suffer degradation attributed to PDL generally through transients and through noise. Transient changes in the polarization couplings along a fiber route cause a transient change in the received power of a polarized signal, which cause errors in the receiver. Furthermore, Amplified Spontaneous Emission (ASE) noise is generally unpolarized, and so PDL along an optical fiber route can attenuate the polarized signal and not attenuate the ASE noise travelling with that signal. This will further impair the optical signal-to-noise ratio. It should be noted that the ASE noise is actually polarized by this process which reduces the effect of the noise upon the receiver, but downstream PDL tends to map much of the orthogonally polarized ASE back into the same polarization as the signal.
Undersea systems generally use polarization scrambling to mitigate the effects of PDL as disclosed in U.S. Pat. No. 5,416,626, entitled xe2x80x9cOptically Amplified Communications Systemsxe2x80x9d, which issued on May 16, 1995. It is further known to attempt to minimize the PDL introduced by an optical device by improving the design of that device, or controlling the temperature of that device to a fixed value. However, in order to mitigate the deleterious effects of PDL inherent in an optical link, it is, therefore, desirable to be able to accurately compensate PDL dynamically. For WDM communications systems, this functionality must be implemented across a wavelength range that encompasses the optical signal traffic.
Various systems have been proposed for addressing this requirement for broadband dynamic PDL compensation. A typical example is described in xe2x80x9cDemonstration of In-Line Monitoring and Dynamic Broadband Compensation of Polarization Dependent Lossxe2x80x9d (L.-S. Yan, Q. Yu, and A. E. Willner, paper We.P.38, ECOC""2001). In this system, broadband PDL compensation is achieved by demultiplexing the optical signal traffic to separate each channel signal into a respective parallel optical path. The PDL of each of the separated channel signals is then independently measured and compensated, in parallel, and the thus xe2x80x9cPDL-compensatedxe2x80x9d channel signals subsequently multiplexed back together.
A limitation of this approach is that WDM systems that achieve high spectral efficiencies (e.g., better than about 0.3 bits per second per Hz) generally suffer significant distortion penalties for each multiplexing and demultiplexing function. In addition, per-channel PDL compensation inherently introduces xe2x80x9cdeadbandsxe2x80x9d between channels. Within these deadbands, optical signals cannot be transmitted and PDL cannot be compensated. This tightly ties the PDL compensation system to the particular wavelength plan of the communications system, which is undesirable.
A known method of broadband PDL compensation that avoids deadbands is to impose a selected PDL across a wavelength band of interest (e.g. 5-6 nm wide). However, PDL can exhibit a strong wavelength dependence. Accordingly, the imposed PDL will normally be selected to compensate an average PDL within the wavelength band. While this approach avoids undesirable deadbands, it can only compensate a portion of the PDL within the wavelength band, leaving at least some PDL un-compensated.
Another method of reducing the accumulation of PDL in a optical fiber link is depolarization of the optical signal traffic. Such methods are taught in U.S. Pat. No. 6,205,262, for example. However, depolarization does not permit any dynamic equalization of PDL across a spectrum of channels.
Accordingly, a system capable of implementing broadband dynamic PDL compensation system, independently of a channel plan of the communications system, remains highly desirable.
An object of the present invention is to provide a system for dynamic broadband compensation of polarization dependent loss in an optical communications system.
Thus, an aspect of the present invention provides a method of compensating polarization dependent loss (PDL) in a wave division multiplex (WDM) optical communications system. In accordance with the present invention, a performance parameter is monitored at a predetermined monitoring point. The performance parameter is indicative of respective channel PDL for each one of a plurality of channels of the optical communications system. A error function is calculated as a function of wavelength across a wavelength spectrum of interest, using the measured performance parameter values. Finally, a broadband PDL compensator is controlled based on the calculated error function.
Another aspect of the present invention provides a system for compensating polarization dependent loss (PDL) in a wave division multiplex (WDM) optical communications system. The system comprises: a monitor; a signal processor; and a controller. The monitor is designed to measure a performance parameter indicative of respective channel PDL for each one of a plurality of channels of the optical communications system. The signal processor uses the measured performance parameter values to calculate an error function as a function of wavelength across a wavelength spectrum of interest. Finally, the controller uses the calculated error function to control a broadband PDL compensator.
The performance parameter may be directly or indirectly indicative of PDL. Directly indicative performance parameters include: directly measured polarization direction of each channel; and measured optical power levels of each mode of each channel. Indirectly indicative performance parameters include channel bit error rate (BER) and Signal to Noise (S/N) ratio.